


For Science

by shiphitsthefan



Category: Supernatural
Genre: Alternate Universe - College/University, Asexual Castiel, Asexuality Spectrum, Bisexual Dean Winchester, Bottom Castiel, Comeplay, Declarations Of Love, Demisexual Castiel, Dom/sub Undertones, Dorks in Love, Established Relationship, Everything is Beautiful and Nothing Hurts, Facials, Fluff, Fluff and Smut, M/M, Nerd Castiel, Nerd Dean, Nerdiness, Rimming, Science, Top Dean
Language: English
Status: Completed
Published: 2015-05-21
Updated: 2015-05-21
Packaged: 2018-03-31 15:34:53
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 6,105
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/3983443
Author URL: https://archiveofourown.org/users/shiphitsthefan/pseuds/shiphitsthefan
Summary: <blockquote class="userstuff">
              <p>“Think of it like an experiment," says Dean.  "You’re testing a hypothesis as to whether or not a desirable response can be achieved through the stimulation of the anus via the application of a willing volunteer’s muscular hydrostat.”</p><p>Cas raises an eyebrow.  “Are you actually trying to use the scientific method to talk me into letting you <i>lick my asshole?”</i></p>
            </blockquote>





	For Science

**Author's Note:**

> I filled this for the [Destiel Smut Brigade](http://destielsmutbrigade.tumblr.com) [Daily Drabble dialogue prompt for May 18th](http://destielsmutbrigade.tumblr.com/post/119272389913/may-18th-daily-destiel-drabble).
> 
> Given that this clocked in at 6100 words, I think it's fairly obvious that I have difficulty writing drabbles.
> 
> Thanks to [betty days](http://archiveofourown.org/users/sadrobots/pseuds/betty%20days/works) for being my beta for life, and a damn good one at that. Thanks, too, to [Eveanyn](http://archiveofourown.org/users/Eveanyn/works) for supporting me while writing my first asexual spectrum character. You guise are the best.
> 
> I found [this guide](http://aroacereads.tumblr.com/post/107831665493/writing-demisexual-characters-without) by [AroAceReads](http://aroacereads.tumblr.com/) incredibly helpful while writing this story. If you're interested in writing a demisexual character, I highly recommend it.
> 
> Please do not repost/copy/duplicate this work to other sites. That's called theft.

"It's not as bad as it sounds," Dean says, poking his head around the shower curtain to talk to Cas.  Dean’s hands are in his hair, and he continues to lather away.

“Well it sounds positively revolting,” Cas replies as he finishes drying off and wraps the towel around his hips.  “Absolutely unhygienic.”

“Do you have any idea how many germs are in our mouths, Cas?  I mean, kissing me, sticking tongues in mouths, now _that_ is what should disgust you.”  Dean winks playfully at Cas as he ducks back behind the curtain to rinse.

Cas folds his arms across his chest and leans back against the doorjamb with a frown.  “That’s hardly the same thing.  You don’t, ugh,” he shudders, “ _excrete_ from your mouth.”

Dean has the audacity to bust out laughing.

“I’m serious,” says Cas.  “I mean, tongues in mouths, acceptable.  We brush our teeth.”

“Knowing you, you’d scrub yourself silly beforehand, anyway,” Dean says as he reaches a hand out for the soap.  “And what about cocks?  What about the piss that comes out of them?  That’s a waste product, too.  You never seem to complain about me wanting to suck you off.”

Cas scoffs and rolls his eyes.  “I keep myself meticulously clean, and my cock in your mouth doesn’t put you in proximity of _Escherichia coli.”_

Dean sighs wearily.  “Okay, dude, seriously, I don’t have Gene Simmons’ tongue, and we’ve both taken anat & phys so you know how butts work.  Risk is minimal.”

“But it’s still present.”

“So is the flu, but you don’t stop breathing because of it.”  Dean trades the soap for a bottle of conditioner, racing to finish his shower before the ancient water heater in their rented off-campus house decides to finish it for him.  “Think of it like an experiment.  You’re testing a hypothesis as to whether or not a desirable response can be achieved through the stimulation of the anus via the application of a willing volunteer’s muscular hydrostat.”  Dean sticks his head around the curtain again and grins.

Cas raises an eyebrow.  “Are you actually trying to use the scientific method to talk me into letting you _lick my asshole?”_

Dean’s grin falters.  “But you love it when I talk nerdy to you.”

“I just don’t understand why you would want to do it in the first place,” Cas admits, and it earns him a fond smile.

“Because rimming feels awesome,” says Dean, “and I want to make _you_ feel awesome.”

Cas smiles back, Dean’s ever-present enthusiasm for finding ways to please him momentarily pushing aside his disgust at the suggestion.  He opens his mouth to ask how, in God’s name, Dean ever thought that his self-described neat-freak of a boyfriend would ever be interested in such a thing when they’re interrupted by banging on the door.  Cas jumps away from it, nearly startled out of his towel.

“Jesus Christ, Dean!” shouts Jo, punctuating her words with the hollow _thunk_ of her fist against the door.  “How much of a princess are you?  You can’t be that dirty when you spend all day in the library!”

“I’m conditioning!” Dean yells back.  Cas covers his ears as Dean’s voice echoes in the small bathroom.

“Well hurry it up!  Me and Charlie don’t want to wait three hours for the water to get hot again.  We’ve both got lab at eight in the stupid morning.”

Dean chuckles.  “Too late for that.”

“God _dammit,_ Dean!” Jo screeches.  “I bet you’re still standing there with conditioner on your head, too!”

“Hey, the bottle says to leave it for three minutes; I don’t write the labels, Jo.”

Jo makes a frustrated noise and stomps off and back up the stairs, the walls shaking slightly with her ascent.

Dean rinses quickly, his teeth chattering in the onslaught of cold water, and then shuts off the shower.  Cas is waiting for him with the other towel, and Dean steps out and into Cas’ fluffy embrace.  He kisses Cas’ forehead as he wiggles his arms up and out of the towel.

“I’m sorry if I made you uncomfortable,” Dean says as he rubs at his torso through the towel, trying to dry off without having to uncover.  “I wasn’t trying to be pushy about it.”

Cas shrugs his acceptance.  “I simply don’t see why that would be a desirable act to perform on someone or to have performed on one’s self.  I understand why anilingus would be pleasurable, anatomically-speaking, but…  Well, the stigma associated with it, Dean.  How is that not off-putting for you?”

“You know me,” says Dean, sliding the towel down his body to dry his legs, one foot propped on the counter so he can still look over at Cas.  “I go with what feels good.”

“I just don’t think I could relax enough for it to ‘feel good’.”

“And that’s okay,” Dean continues as he switches legs.  “It’s not for everyone.  I only wondered if you might be interested.”

Cas worries his lip and traces the rosary tattoo that curves up and around his right forearm.  “You seemed very…  Insistent.”

Dean sighs again.  “I kinda miss rimming someone, not gonna lie.”

“You find our sex life unfulfilling?”  Cas traces his tattoo a little more quickly; it’s a nervous habit he developed almost as soon as he’d had the damn thing done when he turned eighteen.

“Fuck, no, Cas,” Dean says, immediately dropping the towel and taking Cas’ face in his hands.  “Don’t ever think that.  You know I’d rather have you, even if we never had sex of any kind ever, ever again.  If I’d known it was gonna make you feel inadequate, I’d have never even brought it up.”

“I want to be enough for you,” Cas says quietly, “and if that’s something you want to do then—”

“You _are_ enough,” says Dean, tracing Cas’ bottom lip with his thumb.  “You _were_ enough before we ever started doing more in bed than cuddling.”

“But I shouldn’t—”

“—do anything you aren’t comfortable doing,” Dean finishes for him. “If you think it’s gross and icky, then we won’t.  That simple.”  Cas nods, so Dean smiles, releasing his face, grabbing his towel off the floor, and opening the door.

“I’ll think about it,” Cas says as he follows Dean upstairs.

Dean turns his head to reply and gets as far as, “Whatever you want to do,” before running smack into Jo coming down the stairs.

“‘Bout time you got ou— _Dean!”_  Jo throws her towel at Dean’s crotch and covers her eyes.

“What?” Dean asks, shoving past her.

“For shit’s sake, cover your shame!  The only person in this house interested in seeing your dick is Cas.”

“I am appalled by your double standards, Jo,” says Dean in mock offense.  “Charlie walks around naked all the damn time.”

“Only because I need an anti-static work environment and you know it, Dean,” chimes Charlie’s voice from the living room.

“Yeah,” seconds Jo.  “So what’s your excuse?”

“I need a constant line of sight at his naked ass,” says Cas.  “It’s for science.”

“What the hell are you studying?”

“Newton’s first law,” Cas deadpans.  “You’re proving quite the external force, Jo, so if you don’t mind...?”

Jo gapes as Cas gently pushes her aside, and Dean laughs all the way upstairs.

* * *

Cas told Dean he’d think about it, and he has been.

All day, in fact.

Cas thought about it over breakfast, watching Dean fight with Jo over the last bowl of Froot Loops like a couple of kindergartners.  He thought about it during organic chemistry while he was looking for amino acids on the chromatogram.  He especially thought about it in the middle of art history, but he always thinks about art-unrelated topics in the middle of art history, because the class bores the ever-loving shit out of him.

Which brings him uncomfortably back around to the focus of his thoughts.

Assholes.

Tongues therein.

_And he just doesn’t get it._

To be fair, Cas didn’t really get sex at all for the vast majority of his twenty-two years.  When his friends started talking more about which female cheerleader was hottest, Cas’ first assumption had been that he must be gay, because he wasn’t attracted to any of them.  It had been a safe assumption, at first, until his boyfriend asked him after homecoming if they were ever going to do more than kiss and hold hands.

“Aren’t you attracted to me?” he’d asked Cas.

“I like you,” Cas had replied with a frown.

“But don’t you ever want to have sex?”

“No,” Cas had said.  “Is what we have not enough for you?”

And, of course, it hadn’t been.  Cas had come home more confused than ever.  He liked boys.  He wanted to be in a relationship with a boy.  Cas had assumed that meant he was attracted to boys.  Having someone he cared about and was in a relationship with ask him so bluntly how he felt had opened his eyes and made him wonder if there was something he was lacking, some facet of his sexuality that was broken.

Thankfully, there was the internet to turn to, and, after a little time and a lot of research, Cas found himself very comfortable referring to himself as asexual.  Maybe everyone else still thought he was flawed or missing out or, as his mother always said, “just hadn’t found the right girl,” but Cas knew he was neither alone nor crazy, and he’d never really cared about the opinions of others, anyway.

Three years later found Cas sitting in his dorm room getting acquainted with Dean, his assigned roommate.  Dean was absolutely everything Cas was not.  He was loud and outspoken; he got along well with his family and made friends easily; he was easy-going and optimistic.  The only things that Cas and Dean shared were their passions for biology, their presidential scholarships, and George Lucas films.

As it turned out, that had been enough to cement a best friendship, and Cas and Dean were soon inseparable.

Well, mostly.  Dean, it turned out, was also a one-night-stand enthusiast.  There were many nights that Cas spent by himself in the dorm sketching, which was fine.  Dean was his best friend, and Cas had absolutely no interest in him sexually.  Besides, Dean’s pursuit of his chosen hobby gave Cas time to pursue his own.  Cas couldn’t understand why Dean felt the need to bounce from bed to bed, but he never questioned it.

Not until Dean asked him why he never hooked up with anyone.

“I simply have no desire to,” Cas had replied.

“No one caught your eye?” Dean asked.

“No, Dean.”

Dean frowned.  “Is there a problem downstairs?”

“No,” Cas laughed, “everything is fully functional, but I don’t experience sexual attraction.”

 _“Ohhh,”_ said Dean.  “You’re ace.”  He smiled and clapped Cas on the shoulder.  “That’s cool.  Leaves more for me.”  And then he’d turned back to reading Plato like he understood and it was the most normal revelation in the world.

Cas blinked.  This was a reaction he was unused to.  “That’s it?”

“Well, yeah, Cas,” Dean said without turning around.

“You aren’t going to ask me why?”

“Is there a reason?”  Dean did turn around now, concern etched all over his face.  “Did somebody hurt you?”

Cas shook his head.  “No, nothing like that.  It wasn’t a choice to not have sex.  I was just made this way.  I don’t feel arousal.”

“Okay,” Dean said, relieved, “good.  Was afraid I was gonna have to look somebody up and beat the shit out of them.”

Several minutes passed before Cas said, “I’m used to people telling me I just haven’t found the right person yet, that’s all.”

“Are you aro, too?”

Cas was flabbergasted.  “How do you know all these terms?   _No one_ ever knows what I’m talking about.”

Dean shrugged.  “I like fucking.  I like sexuality, sensuality.  Seemed important to know as much about all those subjects as I could.”

“That’s a very enlightened view, Dean.”

“Nah,” Dean said.  “Just the one people should have.”  He hesitated, then asked again, “But are you aromantic?”

Cas shook his head.  “No.  Homoromantic.  I would very much like a relationship, and the hand-holding and kissing and embracing that comes along with it, but most people aren’t okay with me.  They want something I can’t give them.”

“You’ll find someone,” said Dean, the corner of his mouth twitching up into a smile.  “I’m sure of it.”

When they came back after Christmas break, Cas noticed that Dean had stopped going out as often.  He stayed in with Cas, either watching movies while Cas sketched or else building models.

“My little brother accidentally found me a hobby,” Dean said.  “He was making dioramas for theatre, and asked me to help.  Turns out I’m pretty good at it.”  Dean really, really wasn’t, but Cas never voiced his opinion.  He enjoyed spending practically every moment with Dean and wasn’t about to say anything that might change that; if Dean wanted to pretend that he liked model-building, then that was alright with Cas.

They were taking a study break before midterms when it happened.  They’d been marathoning the Indiana Jones movies on Dean’s decrepit VCR for the umpteenth time, sitting on Dean’s bed, and Cas had fallen asleep.  He woke up to discover his head on Dean’s shoulder and Dean’s arm around him, and neither of them moved away.

Halfway through _The Last Crusade,_ Cas put his arm around Dean’s waist and asked, “Is this okay?”

“Of course it is,” Dean said.

That was how they’d started dating.  Dean let Cas lead every step of the way, let him decide what he wanted to do.  It was Cas that kissed Dean first, and Dean never pushed for more.  Sometimes Cas worried that he wasn’t fulfilling Dean’s needs—he’d been such a sexual creature before they got together, after all.

“I’m okay,” Dean told him.  “Just need you.”

They were sitting at the library when it happened.  Dean was quizzing him about the War of 1812, of all things.

“What was the forcible act of recruitment into the British navy called?” Dean asked.

“I love you,” Cas replied.

Dean set the notes down very seriously and reached across the table to take Cas’ hand.  “I know,” was all he said.

Their relationship remained much the same, except that they declared their love for each other more often and took pride in disgusting their friend Jo with their, “mushy, gushy bullshit.”  Dean and Cas were both content with things as they were.  When they moved in with Jo and her girlfriend Charlie that summer, they upped the ante, and were often found kissing in the kitchen, or snuggling on the couch.  Charlie always smiled, and Jo always scoffed.

Two years passed, and, once again, Cas and Dean had been in the library.  Cas looked across the table, annoyed with his art history assignment, and became instantly mesmerized by the sight in front of him.  Dean sat, lost in concentration, the end of a pen in his mouth, and all Cas could suddenly think about was what that mouth would feel like wrapped around his cock.

Cas couldn’t breathe.  Arousal burned through him, a licking flame down his spine and straight to his groin that he had never experienced before.  He felt himself start to harden in his jeans, now thinking about what Dean’s dick would feel like if he held it, warm and heavy and thick in his palm.  He’d felt Dean’s erection before, waking up spooned against him or sometimes when they made out.  But not once had he ever been turned on by it.

“Something wrong?” Dean asked him as Cas threw all of his books in his messenger bag at inhuman speed.

“Just feeling sick to my stomach,” said Cas, and it was true.

Cas walked home, but hardly noticed his surroundings.  He was too busy being disgusted with himself.  Everything every well-intentioned or misunderstanding person had ever said to him about his sexuality had been true.  Cas was confused, and wrong, and now, humiliated.  How could he not have known?  He was open about being asexual now, Dean’s support of him giving him the courage to not hide.  What was everyone going to say?  Would they call him a liar?  Would his other asexual friends, like his lab partner, Kevin, reject him?

He’d locked himself in the bathroom as soon as he got home and sat in the shower until it was well past freezing, quietly having a panic attack.  Eventually, Jo had picked the lock, and she and Charlie had drug him out of the shower, gotten him dressed in his pajamas, and sat down in his room for a heart-to-heart.  Cas cried and told them what had happened.

“I really am broken,” he’d said through his tears.

“Cas,” Charlie asked, pulling him into her lap for a hug that bordered on painful, “have you never heard of demisexuality?”  When he shook his head, she continued.  “It means that a person has the possibility of experiencing sexual attraction after forming a close emotional bond with someone else.”

“But why have I changed?”

Jo rolled her eyes a little.  “You haven’t changed, dummy.  You’ve never been close to someone like you’re close to Dean before, right?”

“No, never.”

“So you’ve probably been demisexual the whole time and just didn’t know,” said Jo.  “And that’s okay.  You’re allowed to find out new things about yourself, Cas. ”

“I don’t understand _why,_ though,” Cas had said.  “Will Kevin feel this way eventually, too?  Does this happen to everyone who’s ace?  Were my feelings not valid before?”

“Cas,” started Charlie, relinquishing her death grip a bit, “people have their own experiences.  Not everyone is going to feel the same way you do.  Kevin very well may never feel sexual attraction, and that’s fine.  You have, and that’s fine, too.  We’re all unique.  I mean, I’m a lesbian, right?  Only attracted to women.”

“Yes, Charlie,” said Cas.  “That’s what being a lesbian means.”

“But I could have the potential of being attracted to a transwoman, too.  Would that suddenly make me straight?”

“Well, no,” Cas conceded, “I suppose not.”

“And another lesbian might feel differently about that.  It doesn’t mean that one of us is more gay than the other.  And Jo’s bi—”

“Damn skippy,” said Jo.

“—but that doesn’t mean that if she was dating a guy that she’s suddenly straight, either.  We don’t have to be a different class just because our stats got assigned different points during character creation or were modified later on.”

Jo sighed.  “Charlie, he’s not going to understand that reference.”

“Okay, but the point _is,”_ continued Charlie, “that this doesn’t have to change the way you identify.  It doesn’t change who you are.  You could be asexual with an exception.  You could call yourself demi.  There are allowances for gray areas in sexuality.  It’s a spectrum, an endlessly undefined graph with an innumerable amount of plot points; what it _isn’t_ is a line segment.”

“Regardless of the math, you should talk with Dean about it,” said Jo.

So Cas did.  Dean, as always, had been completely accepting and understanding.  Cas had still been uncomfortable with the idea of sex for a few months afterward.  He hadn’t been aroused again like he was in the library, anyway.  Then, during a heavy make-out session while ignoring _A New Hope,_ Cas had tentatively lifted his hips and pressed his hardening cock up against Dean.

Dean groaned and pushed his own hips down to meet Cas, ground shamelessly against him.  “Fuck, Cas, you sure?”

“Please, Dean,” said Cas breathlessly.  They hadn’t even taken off their clothes, just laid there in bed, rutting against each other.  Dean tangled his fingers in Cas’ hair, pulled his head to the side to kiss up his neck, up his jaw, to lick at the shell of his ear.  Dean grabbed Cas’ ass with his other hand, pushed them impossibly closer together, and Cas had come helplessly for the very first time, making sounds he’d never believed himself capable of making, moaning Dean’s name like it was his only line to life.

Dean came soon after, and then they just lay there, ignoring the cum drying against the insides of their pants and on their skin, content to just touch each other, to kiss lazily through the end of the movie and on past the automatic rewind of the tape.

They’ve only had a handful of sexual encounters since, trading blow jobs or taking each other in hand.  Dean waits for Cas to initiate, to come to him.  He never asks for anything or suggests what they do; Dean seems perfectly happy to live at the will of Cas’ sex drive and research.

At least, Dean’s never suggested anything until last night in the shower.

Which brings Cas back to now, standing behind the counter at work, hoping his apron hides his erection from the eyes of the customers he’s making coffee for.  Charlie’s already happily pointed out the accidental dicks he keeps drawing in the latte foam, which is bad enough.

“Gee,” she teases now that she’s caught him doing it for the fourth or fifth time, “you got an R2 in your pocket, or are you thinking about Dean?”

“Dean,” Cas confesses, trying desperately to turn his latest coffee art into anything but the cock it looks like.  “He asked me to try something strange last night.”

“Oh, God, he finally wants to act on his Spirk fantasy, doesn’t he?”

Cas chuckles.  “No, Dean—”  He lowers his voice to a whisper.  “—he wants to perform anilingus on me.”

“Dude, please,” Charlie says with a shudder, “just call it ‘rimming’.”

“But why would he want to do that?”

Charlie shrugs.  “Probably because it feels good?”

“Have you done it before?” asks Cas.

“Meh.  It didn’t do much for me,” admits Charlie.  “Jo, on the other hand, really gets into it.  She’d probably tell you to jump on that tongue and ride it like a dragon of Pern.”

Cas continues thinking about it throughout his shift, and by the time he climbs into Charlie’s car, he’s made up his mind.

* * *

Dean was right; Cas scrubs himself in the shower like he’s decontaminating after radiation exposure.  He sticks a hesitant finger up his ass for the very first time, because he has to be _absolutely sure_ that he’s clean on the off-chance that he actually enjoys this.  Cas has enjoyed everything he and Dean have done together so far, and he’s certain he wants Dean to rim him, but that doesn’t negate the worry that something truly awful might happen.

Once Cas is certain, he shuts off the shower, throws a towel around his waist, and starts upstairs. He only realizes that he’s completely neglected to dry his hair once he reaches the top, but it’s too late to turn back now.  Cas passes Charlie, perched on a stool in her workroom over a desk littered with the innards of several computers; she has a pair of safety goggles on her face, pliers in one hand and cutters in another, and absolutely nothing else on.  She hears his feet against the creaky floorboards, turns, and gives him an enthusiastic thumbs up.

Cas opens the door to his and Dean’s room.  Dean is sitting in his boxers at his desk, nose buried in a textbook and feet propped up on the corner of the table.

“I’m ready for you to rim me,” Cas announces from the door.

Dean startles and falls backwards in his chair and crashes onto the floor.  He blinks up at Cas upside-down.  “Are you fucking serious?”

“Yes.”

“You aren’t just doing this because I want to?”

“No.”

Cas has never seen Dean move so quickly.  He rolls over and onto his feet and Cas swears Dean’s crossed the room in the space of a blink.  Dean cradles the back of Cas’ head with his hands and kisses him so sweetly, so softly that Cas feels like he could just melt into him.

“Are you sure?” Dean asks against his lips.

“Yes,” says Cas, and he winds his arms around Dean’s neck and kisses him again.

Dean tangles a hand in Cas’ unkempt, wet, shaggy hair.  He skims the other down Cas’ side, drawing slow lazy spirals with his nails and the pads of his fingers.  When Dean reaches the edge of towel, he pushes it down over Cas’ hips, rubs his thumb over a hipbone, and licks his way into Cas’ mouth.

Cas groans into the deepened kiss and pushes his hip into Dean’s waiting hand.  Neither of them have ever been able to truly figure out which one enjoys the sensitivity of Cas’ hipbones more, but right now, Cas is without doubt that it is him.  He moves his hands, dragging his nails roughly down the back of Dean’s neck, across his shoulders, and down his chest while Dean’s tongue caresses and slides over his own.  Cas reaches Dean’s nipples and runs his thumbs gently across them, over and over until Dean finally takes hold of both of Cas’ hips and starts him backwards toward the bed.  When Cas feels the backs of his knees hit the edge, he sits down and lets Dean push him onto his back.

“What do I need to do?” Cas asks once his mouth is again his own.

Dean chuckles.  “You don’t have to do anything but relax, babe.  Let me take care of you.”

“Do I need to roll over?”

“I’ll let you know,” says Dean with a wink, “trust me.”  He leans over Cas, trailing kisses over his collarbones and down his chest, laving one of Cas’ nipples with his tongue before sucking it into his mouth.  “I’m not going to just dive in, Cas,” Dean murmurs against Cas’ skin as he trails his lips toward the other nipple.  He licks it once, then says, “Wanna get you good and warmed up first.”

“You just want to drive me crazy,” Cas says, trying to catch his breath.

Dean looks up at Cas through his eyelashes, not moving his face from Cas’ chest.  “Is it working?” he asks with a smirk.

“I don’t know, Dean,” says Cas with the best glare he can muster, “what does the empirical evidence suggest?”

Dean turns over Cas’ right arm and traces the tattoo with the tip of his tongue, then says, “I’m not sure.  I don’t think I’ve collected enough background data yet.  What do you think?”

“I think if you don’t hurry up this is going to be over before you ever get to test the goddamn hypothesis.”

“Aw, but Cas,” begins Dean, leaning back a bit, “I thought you’d want the topic thoroughly researched.”  Dean drags his nails up both of Cas’ hipbones, and exhales sharply as Cas curses and bucks his hips up off the bed.  When Cas stills, Dean presses a kiss to each hip then quietly asks him, “What do you want?”

It’s usually hard for Cas to answer that question.  Typically, he’s embarrassed by how inexperienced he is compared to Dean.  Tonight, though, he feels different.  Empowered, even.  He feels more like a participant and less like a subject.  Cas always calls the shots, but this time, he’s evaluating an experience as opposed to re-testing theories he had known would work beforehand.

This is an experiment, and Cas is ready to conduct.

“I want you to lick me open,” Cas says, and Dean’s response is a cross between a moan and a heady laugh.

“You sure you don’t want me to lick anywhere else first?” asks Dean before he takes one of Cas’ fingers into his mouth and swirls his tongue around it.  He sucks on it for a minute, watching Cas’ eyes widen as he stares at Dean’s mouth.  Dean hollows his cheeks and hums a little before slowly backing off.

“Your tongue,” Cas says.  “My ass.  Now, please.”

Dean smiles and says, “Turn over,” as he helps Cas do just that.  He doesn’t jump to it like Dean’s sure Cas is expecting, but instead starts mouthing his way slowly up Cas’ spine, kissing his way from Cas’ shoulders and up the back of his neck before Dean pulls an earlobe into his mouth and nibbling it a little.

Cas, much to his own surprise, whines and buries his face into the crook of his crossed arms.

“You’re trying to kill me, Dean,” Cas says as Dean starts licking back down the way he came, drawing a meandering path with the tip of his tongue.

“Nah, babe,” Dean says into the dimple at the base of Cas’ spine, “but I am gonna take you apart.”

Cas moans, turns his head to rest the side of his face on his arms, and emphatically nods his agreement with Dean’s assessment.

“God, you’re always hot as hell,” Dean says as he kneels in the floor at the edge of the bed, “but you are fucking beautiful like this, all pliant and vulnerable and _mine.”_  He gently pulls Cas’ cheeks apart and asks  “And have I ever told you what a fantastic ass you have?”

Cas opens his mouth to reply, but instead takes a sudden, shaky breath as Dean leans in and starts kissing around his hole.  Cas expected this to feel dirty, and had been concerned that he would hate it immediately.  Instead, it’s strangely sweet, and Cas can’t help but relax into it, thinking of all the other places on his skin that Dean’s kissed already.  It puts butterflies in his stomach, and Cas hums in pleasure.

“That’s it,” Dean says, “don’t think about it, just feel.”

The kisses turn into light, barely-there nips that make Cas gasp and shift his hips back and forth upon contact.  The sheets are soft against his cock as it drags with the movement of his hips.  Cas is positive he’s never been more turned on in his life than he is right now, but then, with flattened tongue, Dean licks a slow stripe from his perineum, over his asshole, and up to the top of his crack, and Cas’ breath gets caught in his throat.

“How’d that feel?”

Cas swallows and chokes out, “Interesting.”

“I must not be doing it right,” says Dean.  “Better try that again.”  And he does, drags his tongue impossibly slower, smiling as he goes.  “How about that?”

“Too…  Too many variab _les oh fuck,”_ Cas ends in a groan as Dean blows warmly over his hole, just the barest huff of a breath.

“Should I narrow the scope of the experiment?” Dean asks.  “Will it be easier for you to compile the data?”

“Dean, _please.”_

Dean concentrates solely on Cas’ asshole now, still keeping his tongue flat and stroking it repeatedly, listening to Cas whimper, feeling him tremble.  He doesn’t stop, and soon Cas is alternating between pushing toward Dean’s mouth and humping forward into the mattress, seeking friction.

“Hottest fucking thing I’ve ever seen,” Dean says, pausing the onslaught of his mouth.  “Love knowing that I can make you go crazy like this,” and then he’s back at it again, flicking his tongue repeatedly across Cas’ hole.

Cas would disagree that he’s being driven nuts, however.  As far as he’s concerned, he’s completely mindless, boneless.  Every nerve in his body feels like it’s on fire, and Cas just follows instinct, rubbing his hard, leaking cock into the bed, a whimpering, moaning mess, canting his hips back in search of Dean’s tongue.  He wants to keep Dean’s mouth buried in his ass forever, never wants him to stop, and he’s too aroused to be surprised at how much he fucking _loves it._

He feels the tip of Dean’s tongue start circling his rim, teasing it open slowly.  Dean giving it careful, tiny licks, dipping into his ass a little further each time, opening him wider.  Cas writhes against the sheets, and doesn’t just hear Dean’s responding groan, he feels it inside him.

“Dean,” he finally manages to call out, “Dean, Dean, you’re getting off on this as much as I am, aren’t you?”

“Fuck yeah, I am,” Dean says, sitting up so he can meet Cas’ eyes where he’s watching over his shoulder.  “Fucking love eating you out,” and Cas gets a look on his face that’s as wild as his sex hair.

“Then I want you to masturbate to it,” Cas says.

“You sure that won’t skew the results of the—”

“Fuck the experiment,” hisses Cas as he pushes himself up to his elbows so he can see Dean, “and just do what I fucking tell you.”

Cas is certain that if Dean wasn’t hard as granite before, he is now, because Dean’s grinning like it’s Christmas and he got everything he wanted.  Dean spits into his hand and frees his cock from his boxers.  Cas shifts up onto his knees and holds himself open, and Dean goes right back to work, fucking into Cas’ hole with the point of his tongue.

Cas closes his eyes and resists the urge to reach under his hips and start stroking himself, because he doesn’t want this to end.  Dean’s tongue dips in deeper, thrusts faster as Cas swears and prays and gasps and does his best to hold on to any shred of sanity he can find.  He wants to outlast Dean, wants to test and see if the manipulation of his body will be Dean’s undoing.

Sure enough, Dean starts to lose his rhythm.  He’s just lapping continuously at Cas’ hole now, stroking the inner walls, and it’s wet and sloppy and warm and so, so _good._  Cas hears Dean’s open-mouthed moaning muffled in his ass, and he wonders if there’s spit dripping down Dean’s chin in the same way that it’s dripping down Cas’ balls, if he looks as wrecked as Cas feels, and he suddenly has to know.

“Stop, stop, stop,” Cas calls out, releasing his cheeks and scooting himself up the bed so he can roll back over.  When he does, he stares at Dean whose eyes are completely glazed, mouth still hanging open, moist and glistening from his own spit, and he’s fucking his own fist like his life depends on it.

“Why…”  Dean breathes heavily and sits back on his heels to relieve his knees.  “Why’d you want me to stop?  Thought you were really, _unngh,_ really into it.”

“I was,” Cas says, scrambling back to the end of the bed, sitting there and looking down at Dean on his knees in the floor as he starts to stroke himself, and holy _shit,_ he’s so close.  “But I want to see you come.”

And Dean does, his eyes snapping shut as he pushes himself into his fist one last time and comes all over his fingers.  Cum drips down his hand and onto his thigh as Dean slumps, head lolling and hitting his chin against his chest.

Cas groans and stands up, a man possessed, compelled.  He pushes on Dean’s shoulders; the action forces Dean to swivel his legs out from underneath him so he can lie on his back, propped up on his elbows.  Cas kneels, straddling Dean’s waist, looming over him, and threads his fingers in Dean’s hair.  He yanks Dean’s head up, pulls Dean’s face closer to his crotch.  Dean moans, a small, submissive sound of want, and Cas grips his cock tight with his free hand, pulls himself roughly, and comes all over Dean’s face, painting it with thick, white stripes of cum.

He lets go of Dean’s hair, and Cas slowly maneuvers himself to sit beside his legs in the floor.  Dean follows him with his eyes, but doesn’t move otherwise.

They both stay there panting and staring at each other for a few minutes, and then Dean says, “Holy fucking shit.”

“Yeah,” Cas agrees.

“That was…  Wow.”

“Yeah.”

“I fucking love you,” says Dean, holding his hand out, an obvious request for help getting up off the floor.

“I fucking know, “Cas replies as he grabs Dean’s hand and pulls him up to sit.

Dean smiles as his shoulders heave with his breath.  “So what do you think?”

“I think,” Cas says, “it’s an experiment that bears repeating many, many times.”  He drags a finger across one of Dean’s dimples, gathering the cum clinging there.  Cas looks at it, studies it, and then offers it to Dean.

Dean’s eyes flutter closed again and he licks it off Cas’ finger.

“After all,” Cas observes as he slides over to Dean and into his lap, “it’s for science.”

 

**Author's Note:**

> The accompanying photoset for this story can be found [here](http://shiphitsthefan.tumblr.com/post/119558333094/for-science-by-shiphitsthefan-6-1k-words). If you liked this story, I would greatly appreciate your reblogging it.
> 
> You can find me on my [tumblr](http://shiphitsthefan.tumblr.com/). I also chirp occasionally witty things on [twitter](https://twitter.com/shiphitsthefan).
> 
> Kudos and comments validate my existence. <3


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